Various transmission technologies are available for wireless digital communications. Among the available technologies is Orthogonal Frequency Division Multiple Access (OFDMA). In a typical OFDMA transmitter, a stream of data bits, representing voice or other payload information, is broken into a plurality of parallel data streams. Each of the parallel streams is conditioned and then modulated onto a radiofrequency (rf) subcarrier selected from a set of mutually orthogonal rf subcarriers. The modulated subcarriers are conditioned for transmission, summed, and transmitted.
The subcarriers are sometimes referred to as “tones”. Accordingly, we will here refer to the conditioned signal that is ready to be modulated onto a subcarrier as a “tone input signal.” Likewise, we will refer to the signal recovered in the receiver by orthogonal demodulation of a single subcarrier as a “tone output signal.”
As noted above, each of the parallel streams of data bits will typically be conditioned before it is modulated onto a subcarrier. The conditioning will typically include mapping the data bits to symbols in accordance with a modulation scheme such as BPSK, QPSK, 8PSK, 32QAM, or the like. The conditioning of the input signals may also include, for example, coding designed to introduce redundancy for purposes of error correction, and coding designed to reduce the peak-to-average power ratio (PAPR) across the OFDMA system. The details of the coding of the data and the mapping of the data to symbols are collectively referred to as the “modulation and coding scheme (MCS)”.
A typical OFDMA transmitter will transmit a plurality of symbols in parallel, using a plurality of subcarriers. Such a transmission will occupy a time interval which we refer to here as a “symbol interval,” and will have a duration which we refer to here, without limitation, as the “packet duration”. The group of symbols transmitted in parallel during one symbol interval is referred to here as an “OFDMA symbol.” Further OFDMA symbols will be transmitted in subsequent symbol intervals.
In order to correctly recover the payload data from an OFDMA transmission, the receiver must have knowledge of the MCS, the packet duration, the allocation of subcarriers to tone input signals, and possibly other such information. Such information may be provided in advance, or it may be provided by signaling, for example over special channels designated for control information.
Another known technology useful for wireless digital communications is Code Division Multiple Access (CDMA) technology. In CDMA transmission, as in OFDMA transmission, an orthogonality property makes possible the parallel transmission of plural streams of input data. In CDMA, the orthogonality is provided by “spreading” the input data; i.e., by multiplying the input data in each of the various streams by a respective spreading code. The spreading code associated with each stream is orthogonal to the spreading codes associated with all of the other streams. The conditioned and spread data from a plurality of input streams may be transmitted in parallel as a composite signal over a single frequency band. At the receiver, after rf demodulation of the composite signal, the orthogonality of the spreading codes is used to demultiplex the composite signal.
A number of proposals have been made for incorporating both OFDMA technology and CDMA technology in a wireless system. For example, one such arrangement is described in the herewith commonly assigned U.S. patent application Ser. No. 11/332,643, filed on Jan. 13, 2006 by P. Monogioudis et al. under the title, “Wireless Communications System Employing OFDMA and CDMA Techniques.”
Such hybrid arrangements may offer certain advantages. For example, in an OFDMA system that makes dynamic assignments of orthogonal subcarriers, the number of such dynamic assignments may be reduced by pre-allocating certain of those subcarriers for carrying CDMA transmissions.
Thus, a hybrid OFDMA-CDMA system will have at least one frequency subchannel designated for carrying the CDMA transmissions. Such a subchannel may comprise as little as one subcarrier. More often, however, a CDMA subchannel will comprise a plurality of subcarriers. These subcarriers may be separated and discrete, or they may be contiguous. If they are contiguous, they may span a continuous band of frequency which we refer to as a “CDMA zone.”
The CDMA subcarriers may be used, e.g., to transport control information, or low-rate user data, or both, as well as other types of information.
OFDMA systems are known to be susceptible to inter-carrier interference. For example, imperfect orthogonality among the subcarriers may cause signals transmitted from different users on the reverse link, or uplink, of an OFDMA system to interfere at the base station receiver. When the various subcarriers are received with similar power levels, the inter-carrier interference is generally comparable to, or even less than, the thermal noise in the receiver. Under such circumstances, the inter-carrier interference can generally be neglected.
However, when the number of users on a CDMA subchannel of a hybrid system is small, the received power on the CDMA subcarriers may be significantly weaker than the received power on the neighboring OFDMA subcarriers. In such a case, the inter-carrier interference may no longer be negligible. For example, some studies have predicted levels of inter-carrier interference as high as −5 dB, which is much higher than typical levels of thermal noise. (It should be noted that other factors, such as imperfect estimation of frequency offsets, may aggravate the inter-carrier interference.) When inter-carrier interference reaches significant levels, one undesirable consequence is that the power-control loop may be affected. That is, the power-control loop may seek to improve reception at the base station by boosting the transmit power of the users. However, as power is increased on the reverse-link CDMA subchannels, intercell interference may also increase on these channels. The overall consequence may be reduced system capacity in the wireless network.
Thus, there remains a need for improved methods of reducing inter-carrier interference in the CDMA subchannels of hybrid OFDMA-CDMA systems.